1. Field of the Invention
The present invention relates to an optical system used in a camera or the like, and more precisely, it relates to a shiftable lens system for varying the range (taking range) of an object to be taken, formed on a picture plane such as a photographic film.
2. Description of Related Art
Upon taking a picture, it is known to intentionally shift the principal optical axis (i.e., an optical axis which passes through the center of a picture plane and is perpendicular thereto) from the center of the picture plane (object to be taken). This is known as shift photographing, where a lens system and/or the image plane are (is) moved from a normal position in which a principal point of the lens system is located on a line connecting the center of the object plane and the center of the picture plane, in a direction perpendicular to the principal optical axis, while maintaining the object plane parallel with the picture plane.
Generally speaking, the photographic film plane is fixed to a camera body, and accordingly, the lens system is moved in the direction perpendicular to the principal optical axis to adjust the shift (displacement).
However, in a known shift photographing mechanism in which the whole lens system is movable or shiftable, it is necessary to provide a large and complex shifting device for moving the whole lens system. In particular, in the case of a wide angle lens which is often and advantageously used in shift photographing and which is usually comprised of relatively large diameter lenses, the whole lens system, to be moved, becomes heavy, and accordingly, a larger shifting device is needed.
To this end, it is known to divide the lens system into front and rear lens groups, of which only the front lens group is shifted to perform the shift photographing. The focusing is then effected by the rear lens group. However, in a known wide angle lens, since the front lens group is made of larger diameter lenses than those of the rear lens group, it is still necessary to use a considerably large shifting device.
Furthermore, since the shiftable lens unit is usually made of a special lens or lenses, it is necessary to prepare a special shiftable lens unit separate from the common lenses, thus resulting in an increased manufacturing cost of the camera system.
Furthermore, to incorporate a shiftable lens system in a zoom lens having a zooming variable power lens group, it is necessary to provide a shifting mechanism for moving the whole lens system, including the variable power lens group, in addition to the driving mechanism for moving the variable power lens group. This makes it difficult to realize a camera that is compact and light.
It should also be noted that if a lens shifting mechanism is incorporated in a camera merely to shift the lens system, the center of an image of an object to be taken would be moved relative to the center of the picture plane during the zooming operation.